A microduplex stainless steel (25.7 wt% Cr-6.6 wt% Ni) was investigated to examine grain growth during static annealing and superplastic deformation at 1000° C. The grain size at a constant strain rate of 1×10-4 sec-1 increases according to d ∼8t0.49 where d is the grain size and t is the time (in min) involved in deformation. Under the present test condition, the contribution of both static (time, tS) and dynamic (strain, ε) annealing appear to be significant and can be expressed by d∞0.19ε0.29. While the exponent of the first term is constant, the exponent of the second term may depend on the strain rate. Strain rate sensitivities were evaluated from differential strain rate tests for different initial grain sizes. Both strain rate sensitivity and grain size were noticed to increase with deformation. © 1983 Chapman and Hall Ltd.